Biological nitrogen fixation potential of 423 mungbean lines were evaluated. There were differences in nitrogen fixation components such as nodule number, nodule dry weight, nitrogenase activity in nodules, amount of nitrogen fixed, total amount of nitrogen in the plant and total plant dry weight. Most of the inoculated Bradyrhizobium sp. (cowpea group) strains could nodulate and fix nitrogen with mungbean lines very well. The specificity found between Bradyrhizobium sp. strains and mungbean lines. The nitrogen fixation activity in mungbean as indicated by nitrogenase activity and (15)N Natural Abundance was highest at the young pod developing stage, 41 to 44 days after germination. correlation between nodule number or nodule dry weight and nitrogen activity of nodules were relatively high. Some F(,3) mungbean lines of selected from crossing between the recommended cultivar and high N(,2)-fixing mungbean accessions lines were superior in nitrogen fixation potential to the parents. However, most of the lines performed nitrogen fixation potential between the parents.